Exploration of the Solar SystemMon 7th- Thurs 10th July 2008, Cumberland Lodge, Windsor Selected
topics include: Magnetospheric structure and dynamics; Planetary
aurorae and magnetosphere-ionosphere coupling; Interaction between
planetary atmospheres and plasma flows; Planetary missions: Where have
we been and where do we wish to go?

Annual Conference of the Aerosol Society Mon 7th
- 8th April, University of Leeds Special Themes: Aerosol Modification
of Climate, and Aerosol Transmission and Causation of Disease. Organised
by Ian Ford

Pulsedelectrically detected magnetic resonance of phosphorous (31P) in bulk crystallinesilicon at very high magnetic fields (B0>8.5 T) and low temperatures(T=2.8 K) is presented. We find that the spin-dependent capture andreemission of highly polarized (>95%) conduction electrons by equally highlypolarized 31P donor electrons introduces less decoherence than other mechanismsfor spin-to-charge conversion. This allows the electrical detection of spincoherence times in excess of 100 µs, 50 times longer than theprevious maximum for electrically detected spin readout experiments.

Based on the article by
Beltran, Codella, Viti, Neri, Cesaroni to appear in Astrophysical Journal
Letters

Glycolaldehyde, the simplest
of the monosaccharide sugars.isan organic molecule directly
linked to the origin of life as it can react with propenal to form ribose, a
central constituent of RNA. Until now it had only being detected towards the
centre of our galaxy where conditions are extreme compared to the rest of the
galaxy and unlikely to host stars or planets. In this article we report the
first detection of glycolaldehyde towards a massive star forming region,
G31.41+0.31, some 26000 light years away.

We used the IRAM Plateau de Bure Interferometer in France
at three different wavelengths and detected three transitions of glycolaldehyde. We were able therefore to derive its column
density which was found to be at least 1017cm-2. In order
to investigate whether such density is chemically feasible in the environment
of G31.41+0.31 we used a chemical model of massive star formation to estimate
the abundances of glycolaldehyde and related species. While the formation
routes of this sugar in space are still a matter of debate we showed that not
only it is feasible to produce large amounts of this molecule but that only
small amounts of the basic ‘ingredients’, such as CO, need to be converted in
larger species in order to produce it.

The discovery of
glycolaldehyde may have profound implications as it is the first time a basic
sugar has been detected towards a star-forming region where planets that could
potentially harbour life may exist. This new discovery also suggests that the
production of this key ingredient for life could be common throughout the galaxy.

Before the Cassini mission arrived at Saturn, we knew that the appearance
of Saturn's auroral emissions was quite different in many ways to those of
Jupiter and would probably behave quite differently when responding to changes
in solar wind conditions.

However the Cassini observations have shown us that Saturn's aurorae are
not as simple as a 'hybrid' of Jupiter-like and Earth-like aurorae. On the one
hand, the brightest auroral 'ring' at Saturn we now know arises from an
Earth-like process, associated with the merging of the planetary and solar
magnetic fields. On the other hand, work by Tom Stallard (ex-UCL now at
Leicester) has also detected a much weaker infrared aurora at Saturn (outside
the 'main ring'), produced by a Jupiter-like mechanism (differential rotation
between the planet and its magnetosphere).

Some of our Atmospheric Physics Laboratory at UCL have recently been involved in a collaboration with the Leicester
group, Imperial College and one of the Cassini instrument teams. This work, led
by Stallard, has looked at infrared images from the VIMS (Visual Infrared
Mapping Spectrometer) instrument, which show Saturn's polar auroral regions and
features at unprecedented resolution (down to a few hundred km). These observations show that, while some of the behaviour of the infrared aurorae follows that seen in the ultraviolet region (by theHubble Space Telescope), there are auroral features apparently unique
to Saturn.

For example, a fairly persistent arc-like aurora equatorward of the main auroral oval near Saturnian midnight. This arc could be the equatorward 'jovian-like' auroral emission recently detected through ground-based infrared spectroscopy by the UCL group. However, it is not clear whether this feature is co-located in
latitude with 'jovian-like' features at other local time sectors. There are also large regions of auroral emission across the polar cap of Saturn (the area inside the main oval). In some cases, the large-scale
polar emission appears to be detached from the main ring, something not
previously seen at any other planet. Thus, while our understanding of the currents flowing between the planet's
ionosphere and the magnetosphere has been able to explain someof the large scale auroral behaviour, these new observations indicate that
there remain undiscovered physical processes at Saturn which often produce
aurorae (so far) unique to that planet.

[Posted 20th Nov. 2008]

Image courtesy pf NASA / JPL / Univ. of Ariz.

Saturn's northern aurora glows bluish-green in this color-coded infrared image. The planet's polar cloud patterns are shown in shades of red. Scientists saythat the areas of auroral activity close to the pole shouldn't be there.

Figure A shows a
schematic drawing of a single Co atom bound on top of a Cu atom of the
Cu2N (copper nitride) surface. Through the Cu2N layer the
Co atom is coupled to the electrons in the bulk copper, which can screen the
magnetic orientation of the the localized spin on the atom.

Figure B shows a
topographic scanning tunnelling microscope (STM) image (10 nm×10 nm) of four
Cu2N islands with single Co atoms. The two diagrams at the sides
sketch the adsorption site of the two marked Co atoms (Cu and N atoms are
depicted as yellow and green circles, respectively). The STM can
then be used to study the interaction of the Co atom with its
environment. [after A.F. Otte et. al., Nature Physics, http://dx.doi.org/10.1038/nphys1072]

With over 300
exoplanets now known, the question as to whether such planets harbour moons as
become increasingly asked. At the University College London, astronomers have
now developed a new method for finding moons by watching the wobble of the host
planet.

When an exoplanet transits, astronomers are afforded a brief
snapshot of the planet's position and velocity. The presence of a moon around
such a planet should cause the planet to wobble, resulting in both position and
velocity varying over time. By watching multiple transits David Kipping, of the
Dept. of Physics and Astronomy, has shown that these variations can be measured,
producing a unique exomoon signature. Furthermore, this signal can be analyzed
to determine both the mass and distance of the moon from the planet. The
UCL-based work also finds that current telescopes could find Earth-mass moons
today and Titan-mass moons in the next few years.

The paper has been
accepted for publication in the Monthly Notices of the Royal Astronomical
Society.

Recent progress in the study of positron and positronium induced ionization
is reviewed within the context of the inert atoms. The focus is on experimental
techniques and results, which comprise both integral and differential
cross-sections. Measurements for positronium formation and direct ionization by
positron impact, accompanied by singly or multiply charged ions, are compared
and discussed. First data, integral and differential, on the fragmentation of
positronium in collision with helium and xenon are also presented. Comparisons
with theories and other projectiles are made where possible and future prospects
are considered.

Article available online (from October 12), published in Nature Materials (November issue)

Insulating materials, such as MgO, are widely used as substrates for thin films and metallic clusters and employed as insulating barriers in spintronic devices. In most cases they are polycrystalline yet the ability of boundaries between crystallites to trap electrons is not well understood. In this letter we investigate the electron trapping properties of grain boundaries in MgO and alkali halides by first principles calculations. Our results show that conduction band electrons, which may be introduced by an applied electrical voltage or irradiation, can be trapped at grain boundaries in MgO, NaCl and LiF. We find that the nature of the electron trapping in these negative electron affinity (NEA) materials is unusual in that the electron is confined in the empty space inside the dislocation cores rather than associated with interfacial ions (see figure). We demonstrate that this surprising effect can be explained using a simple model which should be applicable to other NEA materials, such as boron nitride and alumina. These grain boundaries represent novel examples of systems that are capable of confining electrons and may be probed experimentally by transport measurements or using scanning probes. These effects can also lead to electrons escaping from thin films grown on MgO substrates and reduced tunnel barrier heights in magnetic tunnel junctions affecting the performance of electronic devices.

[Posted 23rd Oct 2008]

Brian Martin (UCL) and Graham Shaw (Manchester) have now published the third edition of the textbook 'Particle Physics'

The textbook is written for senior undergraduates, has been published today by John Wiley & Sons (hardback £90, paperback £35). The book comprises of 450 pages and contains considerable new material, including extensive discussions of neutrino physics, CP-violation and the standard model, and more on gauge theories and the Higgs boson.

The fundamental reaction pp -> {pp}_S gamma, where {pp}_S is a proton pair with low excitation energy, has been observed with the ANKE spectrometer at COSY-Jülich for proton beam energies of Tp=0.353, 0.500, and 0.550 GeV. The differential cross sections measured for c.m. angles 0° < theta_pp < 20° exhibit a steep increase with angle that is compatible with E1 and E2 multipole contributions. The ratio of the measured cross sections to those of np -> gamma d is on the 10-3 ­ 10-2 level. The increase of the pp -> {pp}_S gamma cross section with Tp might reflect the influence of the Delta (1232) excitation.

[Posted 13th Oct 2008]

Fig.1 The challenge when measuring the pp -> pp gamma reaction by detecting the two final protons is to separate this from pion production. As the energy increases, the two corresponding peaks overlap more and more, as illustrated here. The shaded area represents the best fit to the gamma events.

Fig.2 These first measurements of high energy bremsstrahlung leading to the nearly-bound diproton show a strong dip when the diproton angle theta_pp is near the forward direction. This behaviour, and the energy dependence of the production, are probably reflections of the virtual excitation of the Delta(1232) pion-nucleon resonance.

Understanding how molecular hydrogen (H2) binds to materials is crucial to designing a new generation of hydrogen storage media for “clean” fuel cell vehicles. By combining neutron scattering studies with first-principles (DFT) calculations, new insight into H2 binding sites in the layered potassium-graphite intercalate KC24 has been gained. The inelastic neutron spectra show features consistent with a single adsorption site, unlike in the similar compound CsC24. Further, H2 is strongly pinned along a single quantization axis and the H2-substrate interaction is characterized by rotational barriers ~100 times greater than in pure graphite. First-principles calculations suggest hydrogen is sited close to potassium ions but fail to account for the underlying symmetry of the experimental H2 orientational potential. This discrepancy disappears once the H2 position is averaged over three positions close to separate ions in each adsorption site, naturally leading to the well-known saturation coverage of ~2H2 per metal atom in this material. Our results imply that H2 storage in metal-doped carbon substrates can be severely affected by quantum-mechanical delocalisation effects.

[Posted 10th Oct 08]

1 October 2008

Prof. Raman Prinja publishes new book entitled ’Stars: A journey through stellar birth, life and death’

Containing more than 100 beautifully reproduced images, this large-format book presents a vivid account of the fascinating evolution of stars, including our own Sun.

The informative and easily accessible text is combined with stunning imagery from NASA and ESA missions to capture the essence of star birth, the extreme activity of stars and their violent demise, from molecular clouds to black holes.

The book begins with an engaging description of stunning nurseries in space that give rise to new stars. The latest scientific discoveries are explained in an easy-to-understand style that is suitable for novices, enthusiasts and experienced observers.

Hubble Space Telescope and Spitzer infrared images reveal exquisite details of not only the star-formation process in our Milky Way galaxy but also in the extreme environments in other distant galaxies.

The final chapters of ‘Stars’ provide unparalleled insights into the death of stars, covering colourful planetary nebulae and the incredible power of supernovae and hypernovae explosions. ‘Stars’ is a thought-provoking and highly readable study of the stellar universe and our place in it.

Professor Prinja is an expert in stellar evolution and dynamics, particularly massive stars and their outflows. He has previously published ‘Wonders of the Planets’ and ‘Visions of the Universe’.

The time-resolved photoluminescence of cyclodextrin-threaded conjugated molecularwires and of their unthreaded analogues was studied to probe suppression of intermolecular interactions by supramolecular encapsulation. On p… it is shown that polyrotaxane dynamics are exponential and independent of concentration, differently from those of unthreaded semiconductors, which displayed instead optical signatures of interchain species. This is fundamental insight into the photophysics of semiconducting polymers, and crucial for the development of organic electronics.

Dr Sarah Bridle, UCL Physics and Astronomy, has just been awarded a Women in Science Fellowship to discuss dark energy and the even darker matter of the gender gap. There is a podcast avaliable through the Guardian website which also features interviews recorded at the jelly banquet at UCL. Guardian Science Podcast http://tinyurl.com/67m8nx

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.

Physics Students are joint poster winners of the 'Stinkfest conference'

Jenny Brookes and Simon Gane are both Physics and Astronomy PhD students in the CMMP/ LCN research groups and are looking at the mechanisms of signal transduction in olfaction.

The poster was one of two winners at the 'Stinkfest' conference: a one-day conference commemorating 150 years since the great stink of London, at UCL on the 17th June this year.

Congratulations to Emmet Farragher, a first year student in Physics and Astronomy, who has been awarded the inaugural Joseph Rotblat essay prize.

This award was launched to honour the centenary this year of the birth of one of the founders of the Pugwash Conferences on Science and International Affairs, Sir Joseph Rotblat. Rotblat, who shared the 1995 Nobel Peace Prize with the Pugwash Conferences, was an ardent advocate of young people and believed that drawing on their creativity and energy could be crucial in creating a better world.

The prize essay analysed the scenarios in which an independent British nuclear weapon could arguably be used, together with an examination of non-nuclear alternatives in each case.

Emmet received his award from Ambassador Jayantha Dhanapala, the President of Pugwash and a past UN Under-Secretary-General for Disarmament Affairs, following a keynote address on the Urgency of Disarmament at an event jointly hosted by British Pugwash and the SOAS Centre for International Studies and Diplomacy.

Searching for the Higgs boson is one of the top priorities of the CERN Large Hadron Collider, which is about to switch on in Geneva this summer. The Higgs is needed in the Standard Model of particle physics to give particles mass, but it has never been observed. The search at energies around 120 GeV, which is the most favoured region from a theoretical point of view, is very challenging, because the Higgs decays to quarks, which produce jets of hadrons, and the backgrounds are very large. It is widely considered that WH and ZH production are poor search channels for this reason (W and Z are the carriers of the weak force). We show that at high transverse momenta, employing state-of-the-art jet reconstruction and decomposition techniques, these processes can be recovered as promising search channels for the standard model Higgs boson around 120 GeV in mass. Not only might this be the best way of inding a Higgs in this mass region, but it also provides the only way at the LHC to measure the ratio of the WH and ZH couplings, which is important in proving you have actually got a standard model Higgs (or not!). The figure shows two simulated peaks in the mass distribution. The lower (green) peak is from the Z boson at 90 GeV, the higher (blue) peak from a Higgs at 120 GeV reconstructed using our new technique.

Prof Ofer Lahav (Astrophysics Group) heads the UK Dark Energy Survey (DES) consortium. The DES project will use a camera which will scan and map 300 million galaxies in order to understand the relation between dark energy and the universe and thus get closer to nailing Einstein's theory on Gerenal Raliativity and the interaction between time, space and gravity.

Gabriela Halmova, J.D. Gorfinkiel, Jonathan Tennyson

Phys. Rev. Lett., 100, 213202 (2008)

The Coulomb repulsion between like charged particles should keep the particles apart. So it was puzzling when experiments performed in ion storage rings suggested that some negatively charged diatomic molecules could temporarily bind an extra electron.

In this paper detailed quantum mechanical calculations on the problem of electron collisions with negatively charge carbon dimers have been performed. These calculations focus on the electron detachment process, where the electron impact removes an electron, since it is this process that was monitored experimentally. Temporarily bound "resonance" manifests itself as two peaks in the cross sections for particular electron collision energies.

The solution of this problem relied on very large calculations which employed a number of theoretical advances recently developed at UCL. The calculations showed that the extra electron can become temporarily trapped by polarisation forces from the C$_2^-$ target. These only occur when the electron approaches close to C$_2^-$. However our calculations also show that nearly all the electron detachments actually arise from relatively distant collisions which can be modelled using a separate, simpler theoretical treatment.

Electron impact electron detachment cross section for C$_2^-$. Dashed curve: calculations for short range collisions which emphasis the peaks associated with temporary binding of the electron; Solid curve: full calculation including the contribution due to long-range collisions; Symbols with error bars: experimental measurement. The red arrow indicates the minimum electron collision energy required to detach the electron.

Following its two and a half year closure for an extensive £22 million renovation, the Royal Institution of Great Britain (RI) announces Professor Quentin Pankhurst as new Director of the Davy-Faraday Research Laboratory (DFRL). Professor Pankhurst was previously Deputy Director of the London Centre for Nanotechnology at UCL (University College London) and has been appointed to undertake research in Healthcare Biomagnetics – the application of magnetic materials to healthcare.

In his new position, from 1st May 2008, Professor Pankhurst will lead an ambitious collaborative research programme to build a new DFRL team of 15 resident scientists, engineers, medics and technologists to tackle major challenges in the field of Healthcare Biomagnetics. In addition, the RI-UCL programme will see at least another 35 scientists making direct use of the new DFRL facilities.

Professor Pankhurst and his team will be working to make a difference for patients the world over by perfecting new ways to sense, move and heat magnets which can be safely and conveniently introduced into the human body in the form of a magnetic ‘ink’. Examples of on-going projects include the development of:

a hand-held scanner that helps cancer surgeons determine the progression of cancer in a patient,

a treatment method for atherosclerosis using a patient’s own healing cells through the use of targeted magnetic force, and

a method for targeting and activating magnetic particles to cause selective localised heating and destruction of cancer cells

Baroness Susan Greenfield, Director, The Royal Institution said: “We are delighted to welcome Professor Pankhurst and his team to the new Davy-Faraday Research Laboratory. This represents the latest chapter in the long and illustrious history of groundbreaking research at the Royal Institution. I am confident the DFRL will once again become a cornerstone of scientific discovery under Professor Pankhurst’s leadership.”

Professor Quentin Pankhurst said: “My goal for the new DFRL is to create a new paradigm for cross-disciplinary applied research that has technology transfer firmly in its sights. The Healthcare Biomagnetics area is one of tremendous promise and opportunity today. It would also be recognised by the laboratory’s forbears: Sir Humphrey Davy was an apprentice surgeon before making his name at the RI for his work on anaesthetics, and Michael Faraday is undoubtedly the father of modern electricity and magnetism.”

A defining tenet of the RI is that it should be actively engaged in innovative research undertaken by its own resident scientists. The DFRL has a long and distinguished 200 year history, including the award of 14 Nobel Prizes, the discovery of 10 elements of the periodic table and the invention of the electric generator.

The Healthcare Biomagnetics programme at the DFRL is jointly funded by a £2.35 million commitment from the RI and a commitment of £1.36 million from UCL from April 2008 until March 2013.

Christopher Hadley

Phys. Rev. Lett. 100, 177202 (2008)

Ever since Feynman first proposed the idea of a quantum Turing machine in the 1980s, the field of quantum information theory has blossomed, promising super-powerful machines solving classically-intractable problems, perfectly secure communication, and shedding new light on fundamental issues such as the interpretation of quantum mechanics.

The power of quantum mechanics to promise so much relies heavily on the resource of entanglement. This is a strong correlation between particles, stronger than allowed under the laws of classical mechanics. Entangled particles, in some sense, lose their individual identities and only truly exist as part of a pair or group.

Entanglement occurs naturally in groups of physical systems; for example, spin chains, which are often used as a model of magnets. The amount of entanglement occurring is often given as an indication of how powerful such a system would be as a quantum computer.

The standard measures of entanglement used at present are a little artificial, in as much as they measure the amount of entanglement that can be extracted from a system if one has an infinite number of copies of the system! This is called the 'asymptotic limit'. A far more realistic measure is given by a quantity called the 'single copy entanglement', which is how much entanglement exists in one copy of the system.

In this paper, I give the first indication that for a particular class of systems (those with a finite gap between the energies of the ground state and the first-excited state), the single copy entanglement is equal to the asymptotic amount! This means that all the entanglement present can be extracted from a single copy of the system, and that the presence of many copies of the system does not convey any advantage. Previously, it was known that for systems that have no energy gap, half the entanglement present may be extracted from a single copy.

I hope that this result will go some way in to understanding the process of entanglement extraction from naturally-occurring many-body systems, and the structure of this particular class of system.

FIG. 1: The VBS state has an interpretation in terms of 'bonds' between spins, where each bond is a maximally-entangled state, and at each bulk spin the state is projected (dotted circles) to the symmetric space of total spin. The single copy entanglement measured is with respect to the partitioning into a block of length L and the remainder.

Methane found on Distant World

Work in the Department on Extrasolar Planets took another leap forward in March when Giovanna Tinetti was amongst the authors of an article in Nature describing the first detection of Methane on a planet outside our solar system. Methane is made of Carbon and Hydrogen and so is an important simple organic molecule - important in that it can take part in prebiotic chemistry, the first steps in the chemistry of life.

The planet on which it was found - the tongue-twistingly named HD189733b -

is unlikely to support life as it is a Gas Giant (like Jupiter) very close to its star. However the fact that Methane has been found on another world increases our confidence that it may also be abundant on planets with a more equable environment - and illustrates that it is practically possible to detect Methane spectroscopically at these distances.

The detection was made using measurements from the Hubble Space telecope, and at the same time the team was able to confirm their previous detection of water on this same exoplanet. It need hardly be said how important water is in our search for life elsewhere in the universe. The planet "transits" - that is passes in front of its star as seen from Earth, and so some of the light comes to us after penetrating through the planet's atmosphere. It is the subsequent absorption of different parts of the host star's light spectrum that indicates the presence of the water and methane.

HD1897233b is 63 light-years away in the constellation Vulpecula, a near-neighbour in Galactic terms. Over 300 extrasolar planets are now known in other star systems and researchers like Tinetti's team are becoming more hopeful that one day we will be able to make measurements of the atmospheres of terrestrial sized planets "out there".

Data compared to modelling results for HD189733b: The observations of the spectrum of the absorption from the planet's atmosphere is shown by the triangles (with their error bars). The blue and orange lines show the attempts to fit this with model results. The blue line is a model which only includes water: if you add methane to the mix the fit is much nearer showing that methane must be present. The crosses show what models give if their results are binned into the same resolution as the data. Besides methane, models with Carbon Monoxide (CO) and Ammonia have been tried but as you can see although these seem to improve the fits in places, the data is a little too noisy to be sure of the inclusion of these other species.

The Transit Light Curve for HD189733b: These curves show the observations from Hubble in two different frequencies of the transit of the planet - that is a small amount of star-light is cut off as it passes in front of the star. (The label "Absorption" for the vertical axis is wrong - it is actually relative intensity of received light normalised to 1 when the planet is off-transit. So we see the difference at maximum transit is 1 - 0.975, that is 2.5% of the light is cut off by the planet. The other figure following this shows the spectral variation of the small part of that 2.5% which is due to the planet's atmosphere.) The different colours here represent the different light bands of the observations: 1.6-1.8 microns in blue and 2.0 to 2.4 microns in red: the red data have been offset by 0.01 so the two data sets don't fall on top of each other but the intensity is actually 1.0 off-transit for the red as it is for the blue. The four "sets" of data correspond to 4 Hubble orbits, and the smaller gaps are because Hubble cannot "see" HD189733 all the time. The "spread" of the data is due to the "noise" in individual observations and the bottom plot shows this spread, for both data sets, in more detail. Time is in minutes (MJD is Mean Julian Date) measured with the zero at the centre of the transit.

Graduate School Poster competition winners

Congratulations to Shaun Thomas for winning the UCL Graduate School competition, and Carolyn Atkins who was awarded the runner up prize. Both Carolyn and Shaun are PhD students in the Astrophysics group.

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of ?c=0.020±0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip. We found that the force that it takes to move an atom depends strongly on the adsorbate and the surface. Our results indicate that for moving metal atoms on metal surfaces, the lateral force component plays the dominant role. Furthermore, measuring spatial maps of the forces during manipulation yielded the full potential energy landscape of the tip-sample interaction.

RAE 2008

The department recently completed its RAE 2008 submission, the details of which can be found on the following links: